Positive Train Control, the automatic braking system that experts say would have prevented the deadly Amtrak derailment in Philadelphia earlier this year, is now active on most of the Northeast Corridor. Next up, the rail line between Harrisburg and Philadelphia.

The portion of the Keystone Corridor controlled by Amtrak is in testing stages now, said DJ Stadtler, Amtrak's Executive Vice President for Operations, and should be live in the first few weeks of January. PTC provides continuously updated information about a train as it travels along a route and will automatically slow or stop it if it reaches dangerous speeds and an engineer fails to slow it.

The Northeast Corridor, more than 450 miles long,  carried about 11.7 million people this year and brought in almost $1.2 billion in revenue. The 100-mile Harrisburg to Philadelphia line carried about 1.3 million this year and made about $38.2 million.

To say the Northeast Corridor and Keystone Service are now, or about to be, PTC compliant is not the whole story, though. Amtrak's PTC system, Advanced Civil Speed Enforcement System, involves equipment installed both on trains and on the rails themselves. Amtrak is only able to install the system on rail controlled by the agency, and there are spans of rail along the East Coast not under Amtrak control

A 57-mile stretch from New York City to New Haven, Conn. on the Northeast Corridor, for example, is controlled by Metro-North and PTC installation is still underway. South of Washington D.C., Amtrak runs on tracks owned by the freight lines Norfolk Southern or CSX. Neither is PTC compliant at this point.

When that will happen is fuzzy. Every major passenger and freight rail in the country was supposed to be PTC compliant by Dec. 31, 2015. That deadline was essentially ignored by most rail providers, and Congress recently passed an extension that moved the deadline to Dec. 31, 2018, though still there are possibilities for extensions beyond that date.

"We are looking to ensure that as soon as each freight carrier is ready we're ready on our side," Stadtler said.